The present invention relates to an improvement in a lead-out electrode of a scanning atom probe.
One type of high resolution microscope utilizing a tunnel phenomenon of electrons is a field emission microscope (FEM), that irradiates electrons from a sharp needle tip for observing a radiated electron image which is enlargedly projected (refer to FIG. 5). The microscope utilizes an electric field radiation phenomenon in which a strong electric field is applied under a vacuum state, and electrons are emitted from a surface of a metallic conductor by passing over a barrier or a surface potential by a quantum-mechanical tunnel effect and has a construction in which electrons are radiated from a surface of a tip of a metal formed in a needle-like shape toward a screen coated with a fluorescent substance by operation of the strong electric field, and an enlarged image of a surface of the irradiated metal is projected on the fluorescent screen.
Although a single atom cannot be observed since the resolution of a FEM is as low as about 1 nm, a work function of a fine crystalline face on a semispherical face of the needle tip from an I-V characteristic between a negative voltage applied to the needle and a radiation current. When the voltage applied to the needle is switched from negative to positive and an inert gas at low pressure is introduced into a chamber, the FEM is operated as a field ion microscope (FIM) and an atomic arrangement at the needle tip can directly be observed. The FIM is provided with a characteristic capable of orderly desorbing a surface atom at the needle tip as a positive ion by an electric field evaporation phenomenon. The phenomenon is utilized also for atomic operation by a scanning tunneling microscope (STM). When the desorbed ion is successively detected and identified, the composition of the needle tip can be analyzed at an atomic level. Based on this concept, there has been developed a composite atom probe (AP) of a mass spectroscopy instrument capable of detecting a single ion and a FIM. The AP is the only apparatus capable of analyzing an electron state, an atomic arrangement and a composition distribution of the needle tip. The electric field evaporation is progressed orderly from a first layer of the surface atomic layer by atomic layer and therefore, there can be investigated a composition of respective layers, a composition distribution of an interface and a change in an electron state.
However, since the semispherical face of the sharp needle tip having a radius of curvature equal to or smaller than 100 nm, constitutes an observed an analyzed area, the sample is constituted by polishing a slender wire having a diameter equal to or smaller than 0.2 xcexcm or an end of an rectangular column having a side equal to or smaller than 0.5 xcexcm by a chemical or electrochemical method or by irradiating and Polishing an electron or ion beam. Only a metallic or semiconductor material can be subjected to such a step and it is not easy to apply the step to a conductive organic material, ceramic, or diamond. Further, in the case of a superlattice structure comprising a multi-layered film having a thickness of about 1 nm having different compositions, a thickness of a total thereof is only about 1 xcexcm even when one thousand layers thereof are laminated. It is almost impossible to polish the superlattice structure by cutting out the superlattice structure in a slender rectangular shape along with a matrix thereof while leaving the superlattice structure at the needle tip. Further, it is not easy to analyze a surface where corrosion or catalytic reaction is progressed as it is. Further, according to research and development of an electron source of a next generation closely arranged with very small needles having a height of several xcexcm in a xcexcm order, which attracts attention, it is necessary to investigate a shape, a radiation characteristic, a composition distribution and operational life of each needle tip. Although AP is optimum for the research, when voltage is applied to the closely assembled needles, although an electric field intensity of the needle tip becomes higher than that on a planer electrode at same voltage, the electric field intensity is far lower than an electric field of a long and sharp single needle tip by a difference in the order and it is not easy to subject atom of the needle tip to electric field evaporation. Further, even when the atom is subjected to electric field evaporation, also an ion evaporated from a contiguous needle tip is incident on a detector and therefore, the composition of the individual needle tip cannot separately be analyzed.
As described above, there is a strict restriction on AP in fabrication and shape of the sample and a field capable of making full use of the characteristic is limited. It is a scanning atom probe (SAP) that is devised in order to break through the restriction.
In order to select a specific needle from closely arranged needles and investigate a tip thereof, an electric field needs to localize to the needle tip. Hence, a very small grounded lead-out electrode in a funnel shape is attached to inside of a cabinet of AP and positive voltage is applied to a planar sample closely arranged with very small needles. Then, a high electric field is generated at a single needle tip disposed right below a hole having a diameter of several xcexcm to several tens xcexcm at a tip of the lead-out electrode and the electric field is localized in an extremely narrow space between the hole and the needle tip. According to a calculation of an electric field distribution by a computer, even in the case of an apex angle of a needle tip of 90xc2x0 and a radius of curvature of a tip of 50 nm, a high electric field required for electric field radiation or electric field evaporation is generated at a needle tip. The fact shows that when there are recesses and projections of about several xcexcm on a flat sample face, a tip of the projection can be analyzed. A surface which is not subjected to a smoothing process, a corroded surface, such a surface of a highly efficient catalyst or the like is normally enriched with recesses and projections and therefore, such a surface can be investigated as it is.
FIG. 6 shows a basic structure of SAP. A sample at a left end of the drawing schematically shows an electric field radiation electron source of a closely assembled arrangement type. When a hole of a tip of a lead-out electrode of a funnel type approaches a needle tip or a tip of a projection on a sample face, a high electric field is generated at an extremely narrow area between the tip and the electrode, and electron radiated from the needle tip projects an FEM image on a screen. Further, when an inert image gas such as helium is introduced into a cabinet and positive voltage is applied to the sample, a high resolution FIM image is projected on the screen. Further, when surface atom is subjected to electric field evaporation by superposing pulse voltage on steady-state voltage or irradiating pulse laser beam to the sample surface, the surface atom evaporated as a positive ion, enters a reflectron constituting a mass spectroscopy instrument by passing through a survey hole at a center of the screen and is successively detected. An analyzed area is an area having a diameter of several nanometers through several tens nanometers at a tip of a projection in correspondence with the survey hole. When the analysis is continued, a change in a composition in a depth direction of the area can be investigated with a resolution of a single atomic layer.
Meanwhile, a lead-out electrode of SAP currently used, is mechanically fabricated by producing a very small projection at platinum foil by deep drawing. There poses a problem that a shape of a tip of an electrode fabricated in this way, constitutes a shape of a large sphere and cannot be miniaturized sufficiently. As a result, there cannot be concentrated an electric field for constituting an object to be analyzed only by a specific projection on a sample and only a specific projection aimed at by ASP cannot selectively analyzed. That is, there poses a problem that a specific projection cannot individually analyzed when projections are closely assembled on a surface.
FIG. 1 shows an image produced by observing a lead-out electrode of SAP which is mechanically fabricated by a scanning ion microscope and the lead-out electrode is formed such that a height dimension of a dome portion thereof is 200 xcexcm and a radius of a spherical face at a tip thereof is about 30 xcexcm. An analyzed position limiting function in this case is about several tens xcexcm in correspondence with a radius of the spherical face at the tip of the lead-out electrode. In order to enhance the analyzed position limiting function, a dimension of the tip of the lead-out electrode may be reduced.
Further, there also are poses a problem that it is not easy to select an analyzed area of a scanning atom probe(SAP). In the case of the atom probe, what constitutes an object of an analysis is, in principle, a projected shape portion of a sample. Even when SAP is of a scanning type, a two-dimensional image is not provided by scanning the probe as in other scanning microscope. Although the lead-out electrode can be scanned two-dimensionally, the scanning is for selecting a proper positional relationship with the projected shape portion and individual analysis is carried out with regard to a single very small projected portion of a sample face. It is necessary to specify the single very small projected portion and position the lead-out electrode. Therefore, as previous preparation, a surface shape of a sample face is grasped beforehand by using a scanning tunneling microscope and an analyzed area is selected, however, thereby, STM needs to prepare other than an SAP apparatus, further, it is necessary to transmit positional information provided by STM as positional information of SAP and the execution is not necessarily easy.
The advantage of the present invention is to provide a scanning atom probe apparatus capable easily executing selection of an analyzed area by providing a technology capable of grasping a shape of a sample face similar to a sample face of a scanning tunneling microscope without separately preparing the scanning tunneling microscope, prior to analysis by a scanning atom probe (SAP), further, capable of analyzing only a specific projection on the sample by confining an electric field formed between the sample and the lead-out electrode to a narrow range by providing an electrode machining technology capable of forming a sufficiently small funnel type shape.
According to the invention, in selecting an analyzed area of a scanning atom probe (SAP), there is adopted a method of using a tip of a lead-out electrode of the SAP as a scanning probe of a scanning tunneling microscope (STM) and selecting the analyzed area by drawing a surface shape of a sample. Further, with regard to the tip of the lead-out electrode of SAP, there is formed an exclusive probe of a needle-like shape by CVD micromachining technology using a focused ion beam or a lithographic method in order to promote accuracy of the scanning probe of STM.
Further, according to the scanning atom probe of the invention, in order to make an electric field formed between a sample and the lead-out electrode uniform and accurate and make accuracy of the microscope high, there is formed a lead-out electrode near to an ideal shape by forming a conical dome of a conductive material at a tip of a conical electrode mechanically formed by CVD machining process using focus ion beam and shaping a tip thereof by sputter etching.
Further, a scanning atom probe system according to the invention is provided with a function by which positional information of STM corresponds uniquely as positional information of SAP by providing a composite electrode formed with a projection constituting an STM probe at a circular-ring-like tip of a lead-out electrode, a display for displaying a surface image of a sample provided by an STM function, means for selecting an area analyzed by SAP on the display and a drive mechanism for moving the sample or the lead-out electrode in correspondence with selected position information from the means for selecting the analyzed area.